They will probably never be white again. It could be a product problem since on parts have greyed. Whites need to be washed with whites only. I wash mine with white blouses, no bleach products and they are white as when first bought. So were my panties until I put them in with a bleach product that is suppose to be safe on colors, turned them yellow. I won't make that mistake again. I love white underwear. Another thing is to wear them only one day. Body sweat and oils will stain them.

In my experience, everything elastic does OK on "warm"...by which I mean 100F or so. When I want something white really dazzling, I'll select Duet's "whitest whites" setting which boosts to 127F. However, 127 begins to take a toll on elastic with repeated washings. I've seen this in my underwear and the corners of my bottom fitted sheets. You'll see it beginning after your first experience. Strongly suspect "sanitary" cycle which boosts to 155F would hurry the deterioration but haven't done it repeatedly so don't know.

I was extremely careful. All clothes were washed and sealed in plastic ziplock xxlarge storage bags. All wool (and I collect cashmere vintage and new) was always kept in special clear plastic drawers from the container store, but to be safe I washed all and sealed in 2 gallon ziplocks for months after. All linens etc.. were washed on super hot water, placed in xlarge ziplocks and only taken out as I used them. My mattress and boxspring were placed in dust mite proof bags upon arrival to the new place. The movers helped me do that. I only took one upholstered chair and that was throughly vacuumed and steamed before being placed in a large black bag and sealed with tape and not opened for a month then re vacuumed. My table was vacuumed carefully. I replace all other furniture. In the end I had placed a cashmere scarf, gloves and hat in one of the plastic drawers without cleaning it first and that was the only place I ever found any damage. I threw out those items as they were damaged way beyond saving and cleaned that one drawer and never had a problem again. I have moved again since then and have never had a problem. I do want to say that I owned very little at that time and was able to move in one trip in a large van. I am not an owner of decor items, though I now have several art pieces. I believe that stuff gives the little buggers places to hide. Also another thing about those little holes in cotton t-shirts, I rarely if ever found them in my shirts 6 years ago, it is only now that I find them. My holes are without a doubt, not carpet beetles. Material quality in the past several years has taken a real nose dive. I currently place all my newer shirts in mesh bags when washing them and that is making a difference. Anything I have bought from the Gap is almost guaranteed to have holes after the first wash, so now I don't buy from the Gap. Good luck with getting rid of those nasty carpet beetles.

Only other thing I can think of is to make decorative triangles out of a coordinating knit and sew them into the side seams...wide end in the waist band and tapering to nothing in the hip....esp since they are exercise gear, you could add that decoration. Or make it out of a matching fabric and wear longer tops so the seams aren't noticeable.